Spirometry is a common test of respiratory function that involves measuring the total volume of air inhaled into the lungs over a respiratory cycle. A spirometry test is usually carried out by having a subject inhale air through a tube connected to an air flow meter that measures the total volume of air inhaled during the inspiration phase of the respiratory cycle. At the termination of the inspiration phase the subject exhales through the tube. A curve is generated showing the air flow as a function of time. The curve is analyzed to obtain one or more respiratory parameters of the subject that are used to assess intrathorax airways obstruction. For example, the so-called “forced expiratory volume in one second” (FEV1) is obtained in a test performed during a maximum effort force expiratory vital capacity maneuver started from total lung capacity. FEV1 is a well characterized test of respiratory function and provides useful information in diseased and normal states. In chronic obstructive pulmonary disease, the level of FEV1 is used to grade the severity of the obstruction.
It is known to attach a plurality of microphones to a subject's chest or back in order to record respiratory tract sounds at a plurality of locations on the body surface. U.S. patent application Ser. No. 10/338,742 published on Jan. 9, 2003, as U.S. Patent Application Publication No. 2003-0139679, and now U.S. Pat. No. 6,887,208 discloses a method for analyzing respiratory tract sounds detected by a plurality of microphones affixed to a subject's back or chest. The recorded sound signals are processed to determine an average acoustical energy {tilde over (P)}(x,t1,t2) at a plurality of locations x on the body surface over a time interval from t1 to t2.